A fiber-reinforced resin is excellent in specific strength, moldability, and the like and the application field thereof has been extended. In automobile components for which mass production is expected, the application range of the fiber-reinforced resin has been gradually extended starting from application to special motor vehicles and specific components and, owing to recent environmental problems and the like, there is a pressing need for weight saving of automobiles, and acceleration of the application of the fiber-reinforced resin to general automobile components has been expected.
In the application of the fiber-reinforced resin to general automobile components, a fiber-reinforced resin using a thermoplastic resin as a matrix reinforced with glass fiber or carbon fiber has been developed and it is required to lengthen the fiber length of the reinforcing fiber contained in a molded article and to increase the content of the reinforcing fiber. For example, PTL 1 proposes a fiber-reinforced flame-retardant thermoplastic resin composition, a molded article, and a method for manufacturing a fiber-reinforced flame-retardant thermoplastic resin composition. The weight-average fiber length of the reinforcing fiber in the resin composition is preferably 3 mm or more and more preferably from 5 to 50 mm. The weight-average fiber length of the reinforcing fiber in the molded article is preferably 1 mm or more, more preferably from 1 to 10 mm, and further preferably from 1.2 to 8 mm. Moreover, it is described that the ratio of the reinforcing fiber having a fiber length of 1 mm or more in the molded article is preferably 30% by weight or more and more preferably from 33 to 95% by weight in the whole reinforcing fiber.
PTL 2 proposes a long fiber-reinforced polyamide resin composition containing (A) 20 to 80% by mass of a polyamide having excellent strength, thermal strength, durability, low-water absorbability, and heat-resistant stability and (B) 20 to 80% by mass of a reinforcing fiber having a weight-average fiber length of 1 to 15 mm. As Examples, in the case of inventive examples in which a glass fiber roving is introduced into an impregnation die, impregnated with a polyamide resin fed from a twin screw extruder, subsequently pulled out, and cut by a pelletizer to produce pellets having a length of 10 mm and injection molding is performed using them, it is shown that a weight-average fiber length of 10 mm in the pellet state becomes a weight-average fiber length of 3.4 to 4.75 mm in a molded article. On the other hand, in the case of comparative examples in which a polyamide resin and glass fiber chopped strands are fed to a twin screw extruder and melt-kneaded, the resulting one is cooled and solidified in a water-cooled bath into a strand form, subsequently pellets having a length of 3 mm are produced, and injection molding is performed using them, it is shown that a weight-average fiber length of 0.27 mm in the pellet state becomes a weight-average fiber length of 0.23 mm in a molded article.
PTL 3 proposes a carbon fiber-reinforced resin composition obtained by blending 10 to 300 parts by weight of carbon fiber (B) and 1 to 100 parts by weight of a non-crystalline resin (C) into 100 parts by weight of a thermoplastic polyamide resin (A) having excellent mechanical properties, surface appearance and the like, particularly excellent tensile strength, flexural modulus, appearance/designing ability, and dimensional stability. In a molded article obtained by feeding the polyamide resin and carbon fiber cut into 6.0 mm (cut fiber) to a twin screw extruder and melt-kneading them, cooling and solidifying the resulting one in a water-cooled bath into a strand form, subsequently producing pellets having a length of 3.0 mm, and performing injection molding using them, it is shown that the weight-average fiber length of the carbon fiber is from 0.24 to 0.27 mm.
PTL 4 proposes a resin injection molded article obtained by injection molding of a thermoplastic resin into which a reinforcing fiber and a particulate solid matter are mixed, wherein the particulate solid matter is defined such that an aspect ratio is from 1 to 5, an average particle diameter is 10 μm or less, and a blending amount is from 0.5 to 5% by weight. There are shown Examples in which molded articles of a glass fiber or carbon fiber-reinforced resin using polypropylene or a polyamide resin as a matrix are produced by three methods.
In the three methods, the first method is a method in which a roving of glass fiber or carbon fiber is impregnated with polypropylene or a polyamide resin by introducing it into an impregnation die, then pulled out, and cut by a pelletizer to produce columnar pellets and injection molding is performed using them. The second method is a method in which polypropylene and glass fiber or carbon fiber are melt-kneaded by means of a twin screw extruder, cooled and solidified in a water-cooled bath into a strand form, and subsequently cut to produce chopped pellets and they are subjected to injection molding. The third method is a method in which polypropylene and roving-like glass fiber are fed to a twin screw extruder and melt-kneaded and the resulting one is fed to an injection molding machine to perform molding. According to test results, it is shown that the reinforcing fiber is less prone to break due to a lubricating effect resulting from the addition of the particulate solid matter and the weight average fiber length of the molded article is from 0.67 to 2.85 mm in the case where the particulate solid matter is added.